In
thermodynamics and
molecular chemistry, the
enthalpy or
heat content (denoted as
H or
ΔH, or rarely as
χ) is a quotient or description of
thermodynamic potential of a
system, which can be used to calculate the "useful" work obtainable from a
closed thermodynamic system under constant pressure.
The term
enthalpy is composed of the prefix
en-, meaning to "put into", plus the
Greek word
-thalpein, meaning "to heat", although the original definition is thought to have stemmed from the word, "enthalpos" (ἐνθάλπος). It is often calculated as a differential sum, describing the changes within
exo- and
endothermic reactions, which minimize at equilibrium.
History
Over the
history of thermodynamics, several terms have been used to denote what is now known as the
enthalpy of a system. Originally, it was thought that the word "enthalpy" was created by
Benoit Paul Émile Clapeyron and
Rudolf Clausius through the publishing of the
Clausius-Clapeyron relation in "
The Mollier Steam Tables and Diagrams" in
1927, but it was later published that the earliest recording of the word was in
1875, by
Josiah Willard Gibbs in the publication "
Physical Chemistry: an Advanced Treatise", although it isn't referenced in Gibbs' works directly. In
1909,
Keith Landler discussed Gibbs' work on the 'heat function for constant pressure' and noted that
Heike Kamerlingh Onnes had coined its modern name from the
Greek word "enthalpos" (ενθαλπος) meaning "to put heat into."
Original Definition
This is the heat change which occurs when 1 mol of a substance reacts completely with oxygen to form products at 298K and 1 atm.
The function
H was introduced by the Dutch physicist
Heike Kamerlingh Onnes in early 20th century in the following form:
»
where
E represents the energy of the system. In the absence of an external field, the enthalpy may be defined, as it's generally known, by:
»
where (all units given in
SI)
Application and extended formula
Overview
In terms of thermodynamics, enthalpy can be calculated by determining the requirements for creating a system from "nothingness"; the mechanical work required,
differs, based upon the constance of conditions present at the creation of the
thermodynamic system.
Internal energy,
, must be supplied to remove particles from a surrounding in order to allow space for the creation of a system, providing that environmental variables, such as pressure (
) remain constant. This internal energy also includes the energy required for
activation and the breaking of bonded compounds into gaseous species.
This process is calculated within enthalpy calculations as
, to label the amount of energy or work required to "
set aside space for" and "
create" the system; describing the work done by both the reaction or formation of systems, and the surroundings. For systems at constant pressure, the change in enthalpy is the heat received by the system plus the non-mechanical work that has been done.
Therefore, the change in enthalpy can be devised or represented without the need for compressive or expansive mechanics; for a simple system, with a constant number of particles,
the difference in enthalpy is the maximum amount of thermal energy derivable from a thermodynamic process in which the pressure is held constant.
The term
is the work required to displace the surrounding atmosphere in order to vacate the space to be occupied by the system.
Relationships
As an expansion of the
first law of thermodynamics, enthalpy can be related to several other thermodynamic formulae. As with the original definition of the first law;
»
This expression is described by the diagram above.
Standard enthalpy changes
Definitions
Standard Enthalpy Change of Combustion
Standard Enthalpy Change of Hydrogenation
Standard Enthalpy Change of Formation
Standard Enthalpy Change of Reaction
A common
standard enthalpy change is the
standard enthalpy change of formation, which has been determined for a vast number of substances. The enthalpy change of any reaction under any conditions can be computed, given the standard enthalpy change of formation of all of the reactants and
products. Other reactions with standard enthalpy change values include combustion (
standard enthalpy change of combustion) and neutralisation (
standard enthalpy change of neutralisation).
Examples: Inorganic compounds (at 25 °C)
» (State: g - gaseous; l - liquid; s - solid; aq = aqueous)
Specific enthalpy
The specific enthalpy of a
working mass is a property of that mass used in
thermodynamics, defined as
where
u is the specific internal energy,
p is the pressure, and
v is specific volume. In other words,
where
is the mass of the system. The
SI unit for specific enthalpy is joules per kilogram.
External results
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